Secondary lithium-ion batteries can be made to provide high energy from a relatively low mass of material. During discharge, lithium ions are formed in a negative electrode material and flow from the negative electrode through an electrolyte medium to a lithium-ion-receiving positive electrode material. This flow of lithium ions provides a corresponding flow of electrons from the negative electrode through an external circuit and an external power-consuming device(s) to the positive electrode in order to power the external device. When the lithium ion content in the negative electrode is sufficiently depleted such that the battery is considered discharged, an external voltage may be applied to the cell to reverse the direction of lithium ion flow. During recharge, the lithium ions flow from the positive electrode, through the electrolyte medium and back to the negative electrode where the ions are reduced to elemental lithium.
Electrolytes of lithium-ion batteries typically comprise one or more lithium salts dissolved in suitable organic solvents or ionic liquids that are substantially free of water. The lithium salts dissociate into lithium ions in the electrolyte solutions and are transported therethrough between the positive and negative electrodes of the battery cells during discharge and recharge.
Suitable electrolytes for lithium-ion batteries should have good ionic conductivity and be capable of conducting lithium ions over the temperature range at which the batteries are expected to operate. The lithium salts should also be sufficiently soluble in commonly used solvents and ionic liquids of lithium-ion batteries. Further, the electrolytes should be thermally stable and chemically compatible with the positive electrode and negative electrode materials.
Numerous attempts have been made to provide electrolytes with a balanced combination of these desirable properties. However, there remains a need for an electrolyte that can effectively function in the thermally, chemically, and electrochemically demanding environment of lithium-ion batteries.